The present disclosure relates to switching DC-DC converters supplying DC voltages to various electronic devices, and more particularly to start-up characteristics of DC-DC converters including synchronous rectifier circuits.
Switching DC-DC converters are used as power supply circuits of numerous electronic devices due to their high-efficiency power conversion characteristics. In general, DC-DC converters convert a DC input voltage to high frequency AC power by switching operation of a main switching transistor, apply the converted power to an inductor, rectify a voltage induced to the inductor by a rectifying means, smooth the induced voltage with an output capacitor, and output the smoothed voltage as a DC output voltage. The DC output voltage of a switching DC-DC converter increases with an increase in a duty ratio, which is a ratio of on-time to a switching period, of the main switching transistor. The control circuit detects a DC output voltage, and adjusts the duty ratio to stabilize the DC output voltage at a target voltage.
When a diode is used as a rectifying means, forward voltage drop causes power loss. Some synchronous rectifier circuits achieve high efficiency with reduced forward voltage drop of the diode by using a transistor such as a MOSFET as a rectifying means similar to the main switching transistor, and turning on the transistor during an off period of the main switching transistor, or at occurrence of a forward voltage during an off period. Such DC-DC converters have often a function called “soft start” of gradually increasing the duty ratio at start-up or gradually increasing a target value of the DC output voltage from 0 V to a target value in normal operation to reduce rapid rising of the output voltage at the start-up and inrush currents occurring accordingly.
In some synchronous rectifier DC-DC converters, a synchronous rectifying transistor is fixed to an off state at start-up. (See, for example, Japanese Patent Publication No. 2003-70238 and Japanese Patent Publication No. H11-220874.) Japanese Patent Publication No. 2003-70238 teaches reducing an overcurrent caused by discharging a residual voltage at an output via the synchronous rectifying transistor, when a voltage remains at the output at start-up in a charger in which a power supply such as a battery is coupled to an output of a DC-DC converter. Even if an overcurrent does not flow, a voltage remaining at the output at the start-up is advantageous in handling the problem that a DC output voltage drops at least once, since the synchronous rectifying transistor allows a current for discharging an output to flow. Japanese Patent Publication No. H11-220874 teaches that an inductor current flows in one direction toward an output at start-up, thereby reducing output oscillation at the start of the oscillation.